The present invention relates to a method according to the preambles of the independent claims in connection with a frequency converter, and a frequency converter.
In certain applications of a frequency converter the thermal stress directed towards the frequency converter varies periodically. Such applications include centrifuges, cranes, metal rollers and elevators. A cyclic load is typically formed of acceleration, smooth driving, braking and of an unloaded state, whereby the power components of the frequency converter are considerably stressed during acceleration and braking.
Dimensioning the frequency converters is carried out conventionally in accordance with the highest temporary temperature or cyclic temperature variation. The temperature of a power semiconductor is not allowed to exceed a certain limit, since there is a danger of definitely damaging the power semiconductor. Dimensioning for periodic loading is carried out using a set of curves provided by the component manufacturer and estimates concerning the duration of an individual load in a periodic load as well as the temporal density of loads. Such a dimensioning allows achieving the reliability of the operation of the apparatus without over-dimensioning the apparatus when the density of loads and the profile of a load remain substantially within planned limits.
A problem with current frequency converters is that when the temperature of the power semiconductor in the frequency converter exceeds the allowed maximum limit while in use, the frequency converter interrupts the operation thereof while still in operation in order to prevent damaging the frequency converter. Then the process controlled by the frequency converter undesirably stops owing to too frequent recurrent periodic use or the increase in temperature caused by the damaged cooling in the frequency converter.